Conventional alkaline electrochemical cells have an anode comprising zinc and a cathode comprising manganese dioxide. The cell is typically formed of a cylindrical casing, but casings having flat surfaces or disk type button cells are also possible. The casing is initially formed with an enlarged open end and opposing closed end. After the cell contents are supplied, an end cap with insulating plug is inserted into the open end. The cell is closed usually by crimping the casing edge over an edge of the insulating plug and radially compressing the casing around the insulating plug to provide a tight seal. An end cap plate in electrical contact with the anode can be attached over the plug to form the negative terminal. A portion of the cell casing at the opposing closed end forms the positive terminal.
Primary alkaline electrochemical cells typically include an anode comprising zinc anode active material, an alkaline electrolyte, a cathode comprising manganese dioxide cathode active material, and an electrolyte permeable separator film between the anode and cathode. (Anode active material is defined herein as anode material which undergoes useful electrochemical reaction upon cell discharge.) The separator may typically be of cellulose or cellulosic and polyvinylalcohol fibers. The anode active material can include for example, zinc particles admixed with conventional gelling agents, such as sodium carboxymethyl cellulose or the sodium salt of an acrylic acid copolymer, and an electrolyte. The gelling agents, which become activated upon contact with alkaline electrolyte, serve to suspend the zinc particles in a gelled medium and to maintain them in contact with one another. Typically, a conductive metal nail inserted into the anode active material serves as the anode current collector, which is electrically connected to the negative terminal end cap. The electrolyte can be an aqueous solution of an alkali metal hydroxide for example, potassium hydroxide, sodium hydroxide or lithium hydroxide. The cathode typically includes particulate manganese dioxide as the electrochemically active material admixed with an electrically conductive additive, typically graphite material, to enhance electrical conductivity. Optionally, polymeric binders, and other additives, such as titanium-containing compounds can be added to the cathode.
Zinc anodes for alkaline cells are conventionally prepared in the form of a slurry which is pumped into the cell's anode cavity. The slurry is typically prepared by forming a gelled electrolyte mixture comprising an aqueous alkaline electrolyte, preferably a gelled aqueous potassium hydroxide. Such gelled electrolyte can be formed, for example, by mixing a gelling agent such as a polyacrylic acid gelling agent with aqueous alkaline electrolyte. A zinc powder blend containing a small amount of surfactant is then typically mixed into gelled electrolyte to form an anode slurry mixture. The anode slurry mixture is conventionally pumped into the cell's anode cavity by a slurry pump. Although the slurry pump is designed to keep air from entering the slurry mixture, the pump nevertheless typically allows small amounts of air to enter the slurry as it is pumped into the cell's anode cavity. This causes small air pockets to develop within the anode mixture pumped into the cell's anode cavity. Such air pockets can be microscopic in size but more typically there are a number of such air pockets which are visible to the naked eye when the anode is examined by simple X-ray photography without magnification. Optionally some additional aqueous electrolyte may be added to adjust the electrolyte composition in the anode mixture after the slurry is pumped into the anode cavity. However, such additional electrolyte does not noticeably reduce the number of air pockets within the anode slurry. The presence of such air pockets within the anode slurry tends to reduce the overall conductivity of the anode because it reduces the level of zinc interparticle contact in the region of the air pockets.
In a commercial cell assembly line such zinc slurry tends to plug the dispensing tube, leading to down time of the assembly line. Also, the storage of large batches of the slurry tends to result in settling or precipitation of some of the zinc particles over time, which can result ultimately in a non-uniform distribution of zinc particles. The zinc particle settling or precipitation could also occur from the gelled zinc slurry already dispensed into the cells, for example, in cases where the cells are stored for long time or experience some shock or vibration. Such precipitation of the zinc particles will cause non-uniform distribution of the zinc within the anode and subsequent loss of electrical continuity. Also pumping the anode slurry into very small dimensioned, particularly, small irregular shaped anode cavities becomes difficult. In such case the slurry mixture must be prepared with more attention given to zinc particle shape and slurry flowability (consistency) which can compromise the final anode from the standpoint of overall conductivity. In small size cells, for example, miniature button cells, the anode can be prepared by dispensing a mixture of discrete zinc particles and gelling agent into cell's anode cavity and the adding aqueous alkaline electrolyte to form the gelled anode. In commercial production this becomes an inefficient and speed limited method of forming the anode for zinc/air cells. It can also lead to nonuniform dispersion of zinc particles within the gelled anode.
The manganese dioxide used in the cathode is preferably electrolytic manganese dioxide (EMD) which is made by direct electrolysis of a bath of manganese sulfate and sulfuric acid. The EMD is desirable since it has a high density and high purity. The resistivity of EMD is fairly low. An electrically conductive material is typically added to the cathode mixture to improve the electric conductivity between individual manganese dioxide particles. Such electrically conductive additive also improves electric conductivity between the manganese dioxide particles and the cell housing, which also serves as cathode current collector. Suitable electrically conductive additives can include, for example, conductive carbon powders, such as carbon blacks, including acetylene blacks, flaky crystalline natural graphite, flaky crystalline synthetic graphite, including expanded or exfoliated graphite. The resistivity of graphites such as flaky natural or expanded graphites can typically be between about 3×10−3 ohm-cm and 4×10−3 ohm-cm.
It is desirable to eliminate the need for preparing the anode as a zinc slurry mixture. Rather, it is desired to prepare the anode in preformed solid form which can be inserted as a dry solid into the anode cavity without the need of pumping a slurry mixture into the anode cavity.